A number of access technologies are presently available for wireless data connectivity. These wireless access products include Wireless Local Loop (WLL), Fixed Wireless Access (FWA), Cellular Packet Data (CDPD), Wireless Local Area Network (WLAN), narrow aperture satellite technologies, Local Multipoint Distribution Service (LMDS), and Multipoint Distribution Service (MMDS). A range of data transfer speeds are supported with these technologies, including multiple or fractional T1/E1 type, asynchronous transfer mode (ATM), digital video, plain old telephone service (POTS), and other types of digital signaling connections.
Of chief interest to the present invention are broadband microwave communications systems, such as local multipoint distribution services (LMDS). Now available in the United States and in other countries, LMDS provides fast, flexible and economical broadband wireless connectivity, with line of sight coverage over a distance range of about 3 to 5 kilometers. Schematically, LMDS systems may employ cellular-like designs to provide transport services over a much broader coverage area using functional partitions.
Typical LMDS equipment consists of both a hub and a subscriber unit. Similar to the deployment of a cellular telephone system or wireless local area network (LAN), the hub is deployed at a central site, and is responsible for coupling signals to one or more subscriber units. The subscriber units are located within radio range of the hub. Both types of LMDS equipment include a radio transmitter and receiver, a frequency converter, and a modem interface. The radio transmitter and receiver allow for transmission and reception of LMDS signals at the proper carrier frequencies in the microwave band. The modem interface provides signaling in a format expected by various wire line transport media. For example, cable modem type signaling may be used to interconnect the LMDS equipment to a computer network using coaxial cables. The frequency converter serves to shift the carrier frequency between the microwave carriers used for the over-the-air signals and the much lower carrier frequencies used for the over-the-cable signals.
Within certain countries, such as the United States and Canada, government authorities have allocated multiple microwave frequency blocks for LMDS service. These frequency blocks generally occupy up to 1 GHz bandwidth for the downstream channel and several hundred megahertz (MHz) of bandwidth for upstream channels. These bands generally lie somewhere within the range of from approximately 24 GHz to 32 GHz, depending upon the country. Depending upon the specific details of frequency allocations for upstream and downstream channels, the design of the solid state circuits can therefore present certain challenges. These requirements must be observed together with the requirement that the transmitter must maintain low phase noise to allow the use of common digital modulation schemes, and receiver amplifiers must have a relatively low noise figure. Furthermore, because LMDS must compete in the marketplace with other signal transmission schemes, such as coaxial cable and fiber, the equipment must be designed for small size and low manufacturing cost relative to what has been typical for 30 GHz radio equipment in the past.
Because LMDS is a duplex system, the subscriber unit typically includes an integrated up-converter circuit and a down-converter circuit. The up-converter shifts the carrier frequency of the modem signals from a baseband or intermediate frequency (IF) up to the microwave frequency carriers needed to propagate LMDS signals over the air. The down-converter performs the inverse function, converting the microwave frequency carriers at which the LMDS signals are received, to an IF appropriate for cable modem signalling. While the up-and down-conversion process can be generally be performed in a single frequency conversion stage in LMDS radios, the percentage difference between some of the frequencies in the conversion process is sometimes small, depending upon the band plan allocated for LMDS service. This ends up dictating the need for narrowband filters which must generally be realized as waveguide cavity bandpass filters. This type of filter represents large size, high cost and, depending on the precise frequency band plan, its loss can become significant when very narrow bandwidths are required.
There would be an advantage if the waveguide filter could be replaced or eliminated entirely. However, the high-Q, narrowband characteristic of the waveguide filter cannot be practically duplicated in other circuit technologies. Therefore, multiple frequency conversion stages may be needed if the waveguide filter is eliminated. Microstrip circuitry realized on modem low-loss microwave substrates can be utilized. While a microstrip circuit approach would add the complexity of additional frequency conversion stages, it would have a cost and size advantage over circuits employing waveguide.
Another cost driver is the low-phase noise local oscillator. Phase Locked Dielectric Resonator Oscillators (PLDRO's) provide excellent phase noise but are relatively high cost and not suitable for integration directly on the planar microwave circuit. Phase Locked Coaxial Resonator Oscillators (PLCRO's) available up to about 3 GHz have relatively good phase noise performance, and are compatible for placement on low cost, compact planar microwave circuit structures.
A radio design which makes use of CRO's in place of DRO's would therefore offer significant advantages. However, the use of a lower frequency local oscillator requires careful planning of how the multiple frequency conversion stages are implemented. For example, there is still a design tradeoff to be made in selecting the optimum number of stages, and the exact frequencies of the local oscillators (LOs) used in each stage. If the difference between the frequency of an LO and the required RF signals is small, fewer stages are needed. This has the benefit of reduced complexity, but requires a higher Q, narrower bandwidth bandpass filter. However, if the difference between the effective LO frequency and the RF signal is larger, although more stages may be needed, lower Q filters can be employed.